A Russian banker who fled from the country to the UK three years ago was involved in the £40,000 purchase of a portrait of Margaret Thatcher auctioned by the Conservative party a year ago.

A firm owned by Andrei Borodin's then personal assistant paid for the black and white picture by artist Darren Baker, whose website says the work was "snatched up" at the glitzy Tory summer fundraising ball in 2013.

The 47-year-old exiled banker was granted asylum in Britain February last year – just four months before attending the ball with his model wife Tatiana Korsakova when the portrait was auctioned.

Electoral Commission filings reveal a donation to the Tories was made by Henley Concierge, a firm registered to a cottage on Borodin's £120m country estate near Henley-on-Thames, Oxfordshire.

At the time Henley Concierge was a new firm, owned and run by sole director Mario Hinterdorfer, who was Borodin's personal assistant. Hinterdorfer is understood to have parted ways with Borodin last October, returning to Austria.

Borodin, Korsakova and Hinterdorfer would have to have been on the UK electoral roll in order to be eligible to personally buy the picture, which qualifies as a political donation. But the picture was bought by a UK company – and businesses trading in the UK are permissible donors.

Political party treasurers must check the true source of donations and, for corporate donors, satisfy themselves that firms are carrying on business in the UK.

Borodin told the Guardian and the the Bureau of Investigative Journalism: "I participated in no conversation on policy or party funding issues nor did I make any donation at the [2013 fundraising] event."

He did not answer questions about Henley Concierge, but said he was invited to the 2013 event, which took place at Old Billingsgate market, City of London. He said: "I was invited by several senior members of the Legacy List, a charity supporting work on the Queen Elizabeth Olympic Park. I had previously donated money to that charity."

Borodin says Russia's authorities, who accuse him of aggravated swindling, are "pursuing an illegal and politically motivated witch-hunt against me". He has argued: "By giving me asylum, Britain has recognised that the Russian investigation against me is driven by politics."

A Tory party spokesman would not confirm Borodin was involved in the Thatcher portrait purchase. He said: "All donations to the Conservative Party are properly and transparently declared to the Electoral Commission, published by them, and comply fully with Electoral Commission rules.With regards to this specific donation, the party fully complied with the Electoral Commission guidance with regards to the permissibility of the donor."

Rules on declaring donations require the cost of the item auctioned to be separated from the price paid. That led to a filing with the Electoral Commission showing the artist had given £12,000 to the Conservatives in goods to be auctioned. The balance of the £40,000 paid to be formally declared by Henley Concierge in July 2013 in relation to the purchase was therefore £28,000.Darren Baker, the artist behind the black and white Thatcher portrait, said the work had been commissioned by Elizabeth Diarferia, head of new business for the Conservative party. According to Baker's website, the artwork was "snatched up [at the 2013 fundraising party] ... for an astonishing £40,000! This was a new record for this outstanding young artist".

This report was the result of a collaboration between the Guardian and the Bureau of Investigative Journalism